Synthesis of Some Conformationally‐Constrained Glutamate Mimics of N‐( 5‐(2‐(2‐Amino‐3,4‐dihydro‐4‐oxo‐5,6,7,8‐tetrahydropyrido(2,3‐d) pyrimidin‐6‐yl)ethyl)thien‐2‐ylcarbonyl)‐L‐glutamic Acid (LY254155).